Fix #96: Add a new Layout for the Admin Area.

I've included a start of a new layout for the Admin Area, but it will
need more work in future.

This fixes the immediate issue, but there's a problem with the Create
Session link in the admin area: its using the Admin Route and so 404s.
We can at least access the admin pages again.
Set the Brand URL to Admin Home.

The link didn't go anywhere, but it should go to the Admin home page.